1. Field
The present invention relates to a method and apparatus for connecting coaxial (or micro coaxial) cables to the bottom or interior side of pads of an external layer or internal layer of a printed circuit board (PCB) in order to improve signal integrity of an electrical circuit. In particular the present invention relates to a method and apparatus for connecting the center conductor of a coaxial cable to the bottom or interior side of pads located on an exterior layer or interior layer of the PCB, while also connecting the coaxial's ground shielding to a ground or power plane of said PCB.
2. The Related Art
Currently, when high speed signals are routed through PCBs, they suffer from signal integrity issues that are associated with the multiple structures through which the signal passes in the PCB and the poor impedance control the PCB structures afford. The signals pass through 90 degree angles as they transition from the horizontal trace layers through vertical vias and thus create reflections and other unwanted effects. Further, the horizontal traces have rough surfaces due to the manufacturing process of the PCBs. Due to the skin effect, this rough surface of the traces creates a longer path for the current to travel and creates a greater loss with frequency. The PCB manufacturing process has inherent limitations in controlling dielectric thickness and conductor geometries. These limitations create relatively large variations in the characteristic impedance of the transmission line.
In addition, high speed vias through a PCB typically require ground vias
surrounding them to provide shielding from adjacent high frequency or power vias which would otherwise create cross talk issues. These additional ground vias create greater routing density issues especially in array patterns such as BGA (Ball Grid array) or LGA (Land Grid Array). The same cross talk issues plague horizontal traces on the PCB as well.
It would be desirable to provide a method and structure for connecting a coaxial or micro coaxial cable directly to the interior side of pads in the PCB that avoids the aforementioned drawbacks.